Is there a way to set an absolute value when weight painting for an armature in 2.81a
Set the Weight to the desired value, the Blend Mode to Mix and the Strength to 1. That will erase everything above your brush and set it to the desired weight.

Alternatively, you can go in Edit mode and select the vertices you want to "flatten" to a value. Then go to the Object Data Properties panel ; under Vertex group, select the desired group, set the desired weight, then hit Assign.